Erik-Jan Bos, a philosopher at Utrecht University in the Netherlands, unearthed research gold with an Internet search. In putting together a critical edition of René Descartes’ correspondence, due out in 2014, he discovered a stolen, never-before-published letter from the seventeenth- century French philosopher and mathematician. In the run-up to Descartes’ 416th birthday on 31 March, Bos discusses the hazards of chasing him down.